This invention relates generally to network cable testing, and in particular to providing a network cable test instrument with a cable assembly to interface with a network.
To meet the increasing demands for installation and testing of local-area networks (LANs), test equipment must quickly and accurately verify the quality of cabling in the networks and diagnose problems. LANs are typically implemented by physically connecting systems devices, such as computers, printers, etc., together using twisted-wire-pair LAN cables, the most common being what is known as a quad twisted-pair data cable. This type of cable is an unshielded twisted-pair type xe2x80x9cUTPxe2x80x9d cable which is 8-wire cable configured as 4 twisted pairs. An industry working group known as the Telecommunications Industry Association (TIA) has promulgated standards for the quality and performance of these cables, such as minimum crosstalk isolation and data throughput rates over a range of frequencies.
One prior art network cable test instrument known as the Fluke DSP-4000 connects to a LAN through a link interface cable, which includes a patch cord that is a quad twisted-pair data cable as mentioned above. In fact, this particular tester has the capability of connecting to a variety of networks and connector types by use of interchangeable modules and patch cord links with different types of connectors. The link interface cable, with its patch cord and connector, is typically the most problematic link in terms of reliability and stability, poor performance and unacceptable crosstalk in testing LAN cables. For this reason, the crosstalk response of the near end connector and patch cord is measured to produce mathematical constants that are subsequently used to subtract the undesired cross talk from the measurement. One process for determining near-end crosstalk is described in U.S. Pat. No. 5,532,603, and a process for determining cross talk in a patch cord is described in U.S. Pat. No. 5,821,760. The mathematical constants are stored as calibration data in the interface module so that when the network cable test instrument is in use in its intended measurement environment, it will portray to the cable installer or network specialist an accurate assessment of the cables under test since undesired performance characteristics such as crosstalk associated with interface link and connector will be subtracted off.
Having interchangeable link interface cables, or patch cords with different connectors, allows testing of different LAN systems, but requires the user of the network cable test instrument to carry them all around from job to job. The link interface cables, which may typically be three to six feet in length, may be coiled up when not in use, but still represent considerable bulk. This may be problematic when several different link interface cables must be taken with the network cable test instrument to each test site.
A major disadvantage of prior art link interface cables is that the electrical characterstics of the quad twisted-pair patch cords change with use, affecting the accuracy of measurements. Even coiling and uncoiling the patch cord results in changes of electrical characteristics which may be relatively slight changes each time but accumulate over time. Certainly, events occurring during the normal course of use such as dropping a heavy object on a quad twisted-pair patch cord, or stepping on it, or coiling it too tight, or kinking it will result in physical changes in the twisted pairs, and consequently, in the electrical characterstics. A serious problem is that the user may not even know that the characteristics have been altered and that the accuracy of LAN measurements is affected.
Link interface cables having shielded quad twisted pairs such as that manufactured by Belden Wire and Cable Company and described in U.S. Pat. No. 5,303,630 provide some measure of reduced crosstalk and interference, but do not solve the problem of accumulated changes in electrical characteristics caused by repeated stress on the twisted pairs.
It would be desirable to provide a link interface cable assembly that remains stable with use and minimizes the foregoing problems.
In accordance with the present invention, a link cable assembly is provided as an interface between a network cable test instrument and a network to be tested.
The link cable assembly includes a link cable having an interface adapter fixedly attached to one end thereof and having an instrument connector for connecting the cable to a test instrument, and one of a number of interchangeable connector personality modules releasably attached to the other end thereof and having a network connector for connecting to a network to be tested by the cable test instrument. The link cable preferably includes a plurality of shielded differential pairs of wire. Each of the plurality of differential pairs of wires comprises two wires arranged in juxtaposition relationship within a dielectric medium, with the wires maintained in constant spatial relationship to provide a nominal 100-ohm characteristic impedance. Shielding is provided to minimize crosstalk and magnetic interference. The plurality of differential pairs of wire are also arranged in juxtaposition relationship within a outer sheath or jacket, resulting in all of the wires being in the same plane, or very close to the same plane. This not only helps in reducing crosstalk, but results in a long lasting and reliable xe2x80x9cflatxe2x80x9d cable that can be flexed or bent without unduly stressing the differential pairs or permanently changing cable performance characteristics.
Calibration data may be stored in either or both the interface adapter and the connector personality module to permit xe2x80x9cpatch cordxe2x80x9d intrinsic return loss to be effectively removed from the cable measurement over a wide range of frequencies. The data link includes an embedded data cable which permits the test instrument to retrieve identification information and calibration data from memory in the connector personality module. Thus, the link interface cable assembly features interchangeability of connector personality modules while always being calibrated up to the network port.
Other features, and advantages of the present invention will become obvious to those having ordinary skill in the art upon a reading of the following description when taken in conjunction with the accompanying drawings.